Portable chemical heaters with flameless operation are desirable for heating food and various other applications requiring a portable source of heat. For example the United States Army now uses a Flameless Ration Heater (FRH) rather than a portable camp-stove to heat the pre-packaged Meal, Ready-to-Eat (MRE) eight-ounce (227 g) field ration. The FRH consists of a super-corroding magnesium/iron mixture sealed in a waterproof pouch (total FRH weight is approximately 22 grams). To operate the FRH, a pouch is opened in which the MRE is inserted, and approximately 58 grams of water is added to a fuel containing portion of the FRH surrounding the MRE to initiate the following reaction:Mg+2H2O→Mg(OH)2+H2 
Based upon the above reaction of the fuel, the MRE temperature is raised by approximately 100° F. in less than 10 minutes. This is equivalent to a heat transfer rate of approximately 0.2 to 0.75 Watts per gram of food heated (the actual value depending on the heat capacity and the exact time lapse of heating). The maximum temperature of the system is safely regulated to about 212° F. by raising and condensing of steam. The specific energy content of the FRH is approximately 13 kJ per gram of dry magnesium and about 1.3 kJ per gram of total device weight including packaging and water. The current FRH, while effective, produces hydrogen gas as a byproduct generating safety, transportation and disposal concerns, and making it less suitable for use in consumer sector applications where accidental misuse could lead to fire or explosion.
While the currently used Mg—Fe FRH is effective at heating rations, another particular drawback with present military FRH technology is that it requires water to activate. The required water, in addition to being heavy and spacious, is typically obtained from a soldier's drinking water supply, which is often limited. Additionally, present systems also require the soldier to inconveniently add the water as an additional step in the process of activating the FRH.
Other self-heating food packaging products are available in consumer products based on the heat of hydration from mixing “quicklime” (calcium oxide) and water (CaO+H2O→Ca(OH)2) which does not generate hydrogen. With water present the peak temperature is similarly limited to 212° F. but even neglecting the weight of packaging and water, the specific energy of the system is low (approximately 1.2 kJ per gram of CaO). Self-contained systems must also provide some means of mixing the segregated reactants adding complexity and bulk. Measurements on some commercial self-heating packaged food products are shown in Table 1.
TABLE 1Food productTotal packageSpecific(net)(gross)energy ofVolumeVolumeheaterWeight (g)(ml)Weight (g)(ml)(kJ/g)Coffee3002955516000.34Beef stew4254818839630.13
While quicklime based heaters may offer greater safety than the Mg based heaters, those heaters which utilize quicklime significantly lower specific energy cause the weight and size of the heater to approach that of the object being heated, reducing portability.
Portable flameless chemical heaters that do not generate hydrogen and with device specific energy content of greater than 0.5 kJ/g are thus desirable. Performance, cost, safety of operation, transportation, storage, and disposal are all desirable requirements of any energy storage/delivery system.
Fuel/air reactions if achieved without flame, can offer an advantage in terms of specific energy per weight, and bulk, since one reactant (oxygen in air) need not be stored in the device. While not typically regarded as fuels, the air oxidation of metals can produce significant amounts of energy as indicated in Table 2. Many common metals including: iron; magnesium; aluminum; zinc; and, tin are classified as combustible.
For comparison purposes, it is noted that the energy content of hydrocarbon fuels is ranked using gross caloric values (GCV); GCV being the energy generated per unit mass on complete combustion. Table 2 shows approximate GCV values for various hydrocarbon fuels or the equivalent enthalpy of formation for the metal oxides of combustible metals. Note for example that the energy content of lithium metal exceeds that of most hydrocarbon fuels and that of aluminum is comparable to alcohol.
TABLE 2FUELHCMetalsReaction ProductsCalorific Value (kJ/g)HydrogenH2O150LithiumLi2O86MethaneCO2 & H2O55KeroseneCO2 & H2O47GasolineCO2 & H2O45-47Fuel OilCO2 & H2O43CoalCO2 & H2O26-40AluminumAl2O331EthanolCO2 & H2O30MagnesiumMgO25WoodCO2 & H2O16-17ZincZnO9IronFe2O38TinSnO2
Some potential advantages of metal fuels include: high density (compact); solid (no spill or leakage); and, solid reaction products (no emission). These attributes combined with high energy content suggests that metal oxidation reactions may be uniquely suited for use in portable chemical heaters.
There presently remains a need for a portable chemical heater, such as for heating portable food rations such as an MRE, that, among other things, utilizes the high energy content of particulate fuels containing metal.